1. Field of the Invention
The invention relates to a facility for steam cracking of hydrocarbons and an implementation process, comprising a step of decoking by controlled injection of solid particles.
2. Description of the Prior Art
The prior art is illustrated by FR-A-1.433.702.
The steam cracking process is the basic process in the petrochemicals industry and consists of high temperature cracking then rapid cooling of a feed of hydrocarbons and steam. The main operating problem arises from the deposition of carbon-containing substances on the internal walls of the facility. These deposits, constituted by coke or condensed heavy pyrolysis tars, which are coagulated to a greater or lesser extent, limit the heat transfer in the cracking zone (coils of pyrolysis tubes) and the indirect cooling zone (effluent transfer line exchanger) necessitating frequent stoppages to decoke the facility.
The applicants have proposed (EP-A-419 643, EP-A-425 633 and EP-A-447 527) a decoking process for use during the operation of steam cracking facilities, by injection of solid erosive particles to overcome coking problems and to obtain continuous or substantially continuous steam cracking (for example with cycle periods of the order of one year).
The solid erosive particles can be injected upstream of the cracking zone of each furnace so that they scour the coke deposited in the pyrolysis tubes, then downstream that of the effluent transfer line exchangers.
Injections are carried out in line, that is to say either, preferably, during the normal operation of the furnace or during periods when the hydrocarbon feeding is interrupted briefly, the furnace then being swept by a delivery of steam and remaining connected to the downstream sections of the facility (primary furnace, cracked gas compression and so forth). This steaming, in the absence of oxygen, can also be used for steam decoking of the furnace tubes when it is carried out over longer durations.
To implement flexible steam cracking, compatible with the use of heavy feeds (gas oil, vacuum distillates) in an existing steam cracking facility designed for cracking naphtha, it was found essential to properly scour the coke deposited in the effluent transfer line exchangers and that decoking during operation of the transfer line exchangers allowed, in an unexpected manner, an existing steam cracking facility to be made compatible with a wide variety of feeds. It was also found, in an unexpected manner, that the coke deposited in the transfer line exchangers was far easier to remove by erosion than the coke deposited in the pyrolysis tubes, and that the process previously proposed for complete decoking of the facility using fine erosive particles was very difficult to implement in a reliable manner with flexible operation under necessarily variable conditions: the configuration of the pyrolysis tubes cannot be adapted for all of the feeds to ensure a correspondence between the local erosive intensity and the local speed of coking (the nature of the coke and its hardness possibly also being greatly varied from one feed to another); on the other hand, with flexible operation, that is with a variable regime of operating conditions, feed type and degree of dilution, the loss of feed and skin temperature of the tubes are no longer reliable indicators of the state of coking of a bundle of pyrolysis tubes, which therefore cannot be determined and controlled in real time.
In order to remedy these disadvantages, a new process linking at least preponderantly chemical decoking of the pyrolysis tubes with at least preponderantly erosive decoking of the transfer line exchangers is proposed. The process thus comprises erosive decoking of the transfer line exchangers, which requires means suitable for introducing the erosive particles into said exchangers.
For most facilities, these transfer line exchangers (TLE's) are of the multi-tube type with an inlet tube plate which can comprise, for example, 50 to 100 cracked gas circulation tubes, each tube being cooled, most often, by circulation of pressurised water in an annular space around said tube.
Each exchange comprises an "inlet cone" with a tapering configuration, connected to the transfer line for the cracked gases, itself connected to the pyrolysis tubes of the corresponding upstream cracking zone.
The technical problem for which a solution, forming the subject-matter of the invention, has been found, relates to the distribution of the particles in the different tubes of the exchanger.
The process does not require a strictly equal division of the quantities of particles in each tube, but relatively low differences in distribution have been sought, and in particular it must be avoided that any one of the tubes receives for example 10% or conversely 10 times more particles than the average value. Indeed, a tube poorly supplied with erosive particles may become blocked because of insufficient decoking, while an over-supplied tube could risk being eroded by the excessive amount of particles.
The distribution of particles must also be carried out without causing substantial erosion of the tube plate of the exchanger.
This technical problem is made much more delicate by several elements and procedural constraints:
1) the very high temperature (typically 850.degree. C.) at the TL exchanger inlets, PA1 2) the intensive coking in this zone, with the risk of blockage of the particle supply lines, PA1 3) the very high circulation velocities, and turbulence in this zone (typically 100 m/s and more) PA1 4) the possibility of impaction of mass fragments of coke detached from the walls upstream, and PA1 5) the relative fragility of the inlet tube plate of the TLE's, having generally a thickness of only approximately 10 mm, with respect to erosion problems. PA1 robust, in order to be resistant to high temperatures, and to the risks of shocks and erosion, PA1 reliable, particularly with respect to the problems of blockage with coke, PA1 high-performance from the point of view of distribution, in local high turbulence conditions, and PA1 not presenting increased risks of erosion of the tube plate. PA1 deformation of the banks, which were too fragile, PA1 successive blockages of the injection at numerous points by coke, probably because of aerodynamic disturbances between the different injection points caused by turbulence and intense recirculation in the inlet cone of the exchanger; these turbulences being able to cause halting of the flow at certain injection points, and blockage thereof. PA1 a) a particle impacter-diffuser (6), comprising solid surfaces disposed opposite the transfer line (1), in the interior of said inlet cone (2), said impacter-diffuser being permeable to gases by means of a plurality of gas passages, but at least 70% opaque seen from said transfer line (1) situated upstream, PA1 b) said injection line (1) opening out into a point of introduction of the particles, situated at a distance L upstream of the impacter-diffuser, not exceeding 2.5 times the diameter D of the tube plate of the transfer line exchanger. PA1 the erosion of the impacter is minimised, PA1 the erosion of the tube plate by the minor fraction of particles which do not rebound on the impacter, but cross it directly, is also minimised. Moreover, the dispersion of the particles by the flows or secondary gaseous jets (there is in fact a double diffusion: directly, that of the gases and after rebounding, that of the particles) improves the spatial distribution of the particles in the different tubes of the exchanger.
The device for introducing and distributing the particles therefore has to be at the same time:
Techniques for injecting and distributing solids in powder form or atomised liquids into a multi-tube exchanger are already known.
These techniques consist of carrying out multi-point injection using injection banks to distribute the solids or the liquids directly in front of the exchanger tubes. These injections are advantageously carried out at a large number of points (for example 15 or 20) or even using a number equal to that of the exchanger tubes) to improve distribution.
The injection banks are generally straight or circular lines comprising nozzles or injection orifices.
This known solution was tried for the application under consideration and rapidly abandoned for several reasons:
The technical problem is in fact very difficult to solve as the constraints are apparently contradictory:
If it is desired that the particles be distributed in a substantially equal manner, it is logical to carry out injection with multi-point spatial distribution, with a large number of points, which presents risks of blockage.
If a small number of injection points are used, these are disposed in particular upstream (for example at 50 times the diameter of the line) so that the particles have time to be distributed properly in the gas. In this case, the velocities of the particles are increased as the particles have time to be greatly accelerated by the high-velocity gas circulation, and there are risks of erosion of the tube plate.
If a central impacter is added, facing the transfer line, in order to avoid direct impact of the particles on the tube plate, the particles go around the periphery thereof, and the distribution of the particles in the downstream tubes is less satisfactory.
One of the objects of the invention is to remedy the disadvantages of the prior art and to address the technical problem mentioned hereinbefore.